Neural stimulation devices are capable of treating various disorders and symptoms of disorders. In the context of neural stimulators, an electrical lead having one or more electrodes is typically implanted near a specific site in the brain of a patient. The lead is coupled to a signal generator that delivers electrical energy through the electrodes to nearby neurons and neural tissue. The electrical energy delivered through the electrodes creates an electrical field causing excitation or inhibition of the nearby neurons to directly or indirectly treat the neurological disorder or symptoms of the disorder.
Experience with deep-brain stimulation (DBS) in the treatment of movement disorders and epilepsy has indicated that, in some cases, patients develop a tolerance or adaptation to the stimulation. A possible cause underlying this long-term habituation may be related to the fact that the nervous system tends to adapt to constant, non-varying inputs and eventually ignores them. In much the same way that tolerance to drugs can develop, tolerance to stimulation may occur with repeated, long-term use.
Conventional stimulus patterns for DBS and other neural-stimulation applications often employ a fixed-frequency, fixed-amplitude, fixed-pulse-width, and fixed-electrode-firing-pattern combination for a given patient. This can result in the same population of neurons being repeatedly activated or inhibited with very little temporal or spatial variation. Such a pattern of activity is highly artificial and non-physiologic compared to the recorded firing patterns of normal neurons.
An example of a method and apparatus employing varying stimulation patterns to reduce the effects of neurodegenerative disorders can be found in U.S. Pat. No. 5,683,422, which is incorporated herein by reference. The '422 patent discloses a closed loop feedback control algorithm for both blocking and facilitating neural activity at a stimulation site. A clinician programs a range of values for pulse width, amplitude, and frequency that the stimulation device uses to optimize the therapy. For blocking neuronal activity, if the feedback sensor values indicate too much activity, the stimulation frequency is increased up to a preset maximum value. If the frequency parameter is at the maximum value, the algorithm next increases the pulse width up to a preset maximum value. Once the maximum pulse width has been reached, the algorithm next increases amplitude in a like manner. Once all parameters reach the maximum value, a notification message is sent to the clinician.
If activation of the stimulation site is desired, the frequency parameter is fixed at a value chosen by the clinician to facilitate neuronal activity. The values from the feedback sensor are used to determine whether neuronal activity is being adequately controlled. Inadequate control indicates that the neuronal activity of the stimulation target is too low. Neuronal activity is increased by first increasing stimulation amplitude up to a predetermined maximum value. When the maximum amplitude is reached, the algorithm increases pulse width to its maximum value. If the maximum parameters provide insufficient stimulation, the clinician is notified.
An additional algorithm readjusts parameter levels downward as far as possible. When parameters are changed, a timer is reset. If there is no need to change any stimulus parameters before the timer has counted out, then parameter values are reduced while still maintaining appropriate levels of the neuronal activity. The various parameter values are reduced until the sensor values indicate a need to increase them.
The '422 patent, however, does not address the problem of development of a physiological tolerance to the stimulation. In the '422 patent, if the neural activity remains constant and the stimulation parameters have been adjusted down to their lowest possible values, no further adjustments are made to any of the stimulation parameters. The stimulation patterns can therefore become regular or constant and a patient can develop a physiological tolerance to the treatment. The stimulation produced by the system of the '422 patent also results in a regular firing pattern (i.e., at a fixed frequency) which tends to differ from the recorded firing patterns of normal neurons, as described above. A need therefore exists for techniques of using stimulation patterns that are less regular and more random in nature, similar to the firing patterns seen in a normal nervous system.
Certain types of therapeutic stimulation may require that a relatively large volume of neural tissue be modulated by the stimulation. This may require a high level of battery power, which in turn may require that implanted neurostimulation treatment devices be explanted to replace a depleted battery. For instance, obsessive-compulsive disorder is often treated with four electrodes placed in a specific brain structure, with each electrode stimulated at a relatively high current, which causes a large drain on an implanted stimulator's battery. There is a need, therefore, for treatment techniques that reduce the amount of battery current required by minimizing the amount of energy utilized, while maintaining the desired therapy efficacy. Similarly, it may not be possible to achieve the desired therapeutic effect without undesired side effects of stimulation when a large volume of tissue is simultaneously modulated. By pseudo-randomly varying the spatial pattern of the modulated neural structures, it may be possible to minimize undesired side-effects while attaining the desired therapy efficacy.